1. Field of the Invention
The present invention relates to a triangular-pyramidal cube-corner retroreflective sheeting having a novel structure. More minutely, the present invention relates to a cube-corner retroreflective sheeting in which triangular-pyramidal reflective elements having a novel structure are arranged in a closest-packed state.
More minutely, the present invention relates to a cube-corner retroreflective sheeting constituted of triangular-pyramidal cube-corner retroreflective elements (hereafter referred to as triangular-pyramidal reflective elements or merely, elements) useful for signs including license plates of automobiles and motorcycles, safety materials of clothing and life jackets, markings of signboards, and reflectors of visible-light, laser-beam, and infrared-ray reflective sensors.
Still more minutely, the present invention relates to triangular-pyramidal cube-corner retroreflective sheeting in which a pair of triangular-pyramidal cube-corner retroreflective elements partitioned by three lateral faces (faces a.sub.1, b.sub.1, and c.sub.1 ; faces a.sub.2, b.sub.2, and C.sub.2 ; . . . ) almost perpendicularly intersecting each other because V-shaped grooves having substantially-symmetric cross sections intersect each other are arranged in a closest-packed state so as to protrude to one side on a common bottom plane (S-S'), faced lateral faces (faces c.sub.1 and c.sub.2) of this pair of triangular-pyramidal retroreflective elements are paired by sharing a base (x), the bottom face (S-S') is a common plane including bases (z and z) of one-side lateral faces (faces a.sub.1 and a.sub.2) and bases (y and y) of the other-side lateral faces (faces b.sub.1 and b.sub.2), and faced lateral faces (faces c.sub.1 and c.sub.2) of the triangular-pyramidal retroreflective elements sharing the base (x) have shapes different from each other, and heights from the bottom face (S-S') up to the apex are different from each other.
Still more minutely, the present invention relates to a triangular-pyramidal cube-corner retroreflective sheeting in which a pair of triangular-pyramidal cube-corner retroreflective elements partitioned by three lateral faces (faces a.sub.1, b.sub.1, and c.sub.1 ; faces a.sub.2, b.sub.2, and c.sub.2 ; . . . ) almost perpendicularly intersecting each other because V-shaped grooves having substantially-symmetric cross sections intersect each other have substantially optically analogous shapes and thereby, have angles .theta. (hereafter also referred to as tilts of optical axes) formed between substantially same optical axes though different from each other in direction by 180.degree. and a vertical line.
2. Description of the Related Art
A retroreflective sheeting for reflecting entrance light toward a light source has been well known so far and the sheeting using its retroreflective characteristic is widely used in the above fields. Particularly, a cube-corner retroreflective sheeting using the retroreflective theory of a cube-corner retroreflective element such as a triangular-pyramidal retroreflective element is extremely superior to a conventional retroreflective sheeting using micro glass beads in retroreflectivity and its purpose has been expanded year by year because of its superior retroreflective performance.
However, though a conventionally-publicly-known triangular-pyramidal retroreflective element shows a preferable retroreflectivity when an angle formed between an axis vertical to a sheet plane (axis passing through the apex of the triangular pyramid of the triangular-pyramidal retroreflective element equally separate from three faces constituting a triangular-pyramidal cube-corner retroreflective element and intersecting each other at an angle of 90.degree.) and entrance light (the angle is hereafter referred to as entrance angle) is kept in a small range. However, the retroreflectivity rapidly deteriorates as the entrance angle increases (that is, the entrance angularity deteriorates).
Moreover, the light entering the triangular-pyramidal retroreflective element face at an angle less than a critical angle (.alpha..sub.c) satisfying an internal total-reflection condition determined by the ratio between the refractive index of a transparent medium constituting the triangular-pyramidal retroreflective element and the refractive index of air penetrates into the back of the element without totally reflecting on the interface of the element. Therefore, a retroreflective sheeting using a triangular-pyramidal retroreflective element generally has a disadvantage that it is inferior in entrance angularity.
However, because a triangular-pyramidal retroreflective element can reflect light in the light entrance direction almost over the entire surface of the element, retroreflected light is not diverged at a wide angle due to spherical aberration differently from the case of a micro-glass-bead reflective element.
However, the narrow dispersion angle of the retroreflected light practically easily causes a trouble that, when the light emitted from a head lamp of an automobile is retroreflected on a traffic sign, the retroreflected light hardly reaches, for example, a driver present at a position distant from the axis of the incident light. Particularly when the distance between an automobile and a traffic signal decreases, the above trouble more frequently occurs because the angle (observation angle) formed between the entrance axis of a light ray and the axis (observation axis) connecting a driver and a reflective point increases (that is, the observation angularity deteriorates).
For the above cube-corner retroreflective sheeting, particularly for the entrance angularity or observation angularity of a triangular-pyramidal cube-corner retroreflective sheeting, many proposals have been known so far and various improvements and studies are performed.
For example, Jungersen's U.S. Pat. No. 2,481,757 discloses a retroreflective sheeting constituted by arranging retroreflective elements of various shapes on a thin sheeting and a method for manufacturing the sheeting. Moreover it is described that triangular-pyramidal reflective elements disclosed in the above U.S. patent include a triangular-pyramidal reflective element in which the apex is located at the center of a base triangle and the optical axis does not tilt and a tilted triangular-pyramidal reflective element in which the apex is not located at the center of a base triangle to efficiently reflect light toward an approaching automobile.
Furthermore, it is described that the size of a triangular-pyramidal reflective element, that is, the depth of the element is 1/10 in (2,540 .mu.m) or less. Furthermore, FIG. 15 in the U.S. patent illustrates a triangular-pyramidal reflective element whose optical axis tilts in the direction to be plus (+) as described later. The tilt angle (.theta.) of the optical axis is estimated as approx. 6.5.degree. when obtaining it from the ratio between the major and minor sides of the base triangle of the illustrated triangular-pyramidal reflective element.
Moreover, the above Jungersen's U.S. patent does not specifically disclose a very small triangular-pyramidal reflective element shown in FIG. the present invention or it does not disclose a size or an optical axis tilt a triangular-pyramidal reflective element must have in order to show superior observation angularity and entrance angularity.
Furthermore, Stamm's U.S. Pat. No. 3,712,706 discloses a retroreflective sheeting in which so-called equilateral triangular-pyramidal cube-corner retroreflective elements whose base triangles are equilateral triangles are arranged on a thin sheeting so that their bottom faces are brought into a closest-packed state on a common plane. Stamm's U.S. patent solves the problems that retroreflectivity is deteriorated and light entrance at an angle of less than an internal total reflection condition passes through an interface between elements and thereby it is not retroreflected by vacuum-depositing with a metal such as aluminum on the reflective surface of a reflective element, mirror-reflecting entrance light, and increasing an entrance angle.
However, because a mirror layer is set on a reflection-side face as means for improving wide angularity in the above Stamm's proposal, a problem easily occurs that the appearance of an obtained retroreflective sheeting becomes dark or a metal such as aluminum or silver used for the mirror layer is oxidized due to penetration of water or air and thereby, reflectivity frequently lowers. Moreover, means for improving wide angularity in accordance with a tilt of an optical axis is not described at all.
Moreover, Hoopman's European Pat. No. 137,736B1 describes a retroreflective sheeting in which a pair of tilted triangular-pyramidal cube-corner retroreflective elements whose base triangles are isosceles triangles are arranged on a thin sheeting while rotated by 180.degree. from each other and whose bottom faces are arranged on a common plane in a closest-packed state. Optical axes of the triangular-pyramidal cube-corner retroreflective elements described in the above patent tilt in the minus (-) direction described in this specification and it is shown that the tilt angle ranges between 70 and 13.degree..
Furthermore, also Szczech's U.S. Pat. No. 5,138,488 discloses a retroreflective sheeting in which tilted triangular-pyramidal cube-corner retroreflective elements each of whose bottom face is an isosceles triangle are arranged on a thin sheeting so that their bottom faces are brought into a closest-packed state on a common plane. In this U.S. patent, optical axes of the triangular-pyramidal reflective elements tilt in the direction of a side shared by two triangular-pyramidal reflective elements paired by facing each other, that is, the plus (+) and minus (-) directions to be mentioned later and the tilt angle is approx. 2.degree. to 5.degree. and it is specified that the size of each element ranges between 25 .mu.m and 100 .mu.m.
Moreover, in the case of European Pat. No. 548,280B 1 corresponding to the above patent, it is described that an optical axis tilts so that the distance (p) between a face including a common side of paired elements and vertical to a common plane and the apex of an element is not equal to the distance (q) between a point at which the optical axis of an element intersects with the common plane and the vertical face and a tilt angle of the optical axis ranges between approx. 2.degree. and 5.degree., and the height from the common plane up to the apex of an element ranges between 25 and 100 .mu.m.
As described above, in the case of Szczech's European Pat. No. 548,280B1, the tilt of an optical axis ranges between +2.degree. and +5.degree. (both included) and between -2.degree. and -5.degree. (both included). In the case of embodiments of the above Szczech's U.S. and European patents, however, only triangular-pyramidal retroreflective elements are disclosed which have optical-axis tilt angles of -8.2.degree., -9.2.degree., and -4.3.degree. and an element height (h) of 87.5 .mu.m.
The above-described conventionally publicly-known triangular-pyramidal cube-corner retroreflective elements of Jungersen's U.S. Pat. No. 2,481,757, Stamm's U.S. Pat. No. 3,712,706, Hoopman's European Pat. No. 137,736B1, Szczech's U.S. Pat. No. 5,138,488, and European Pat. No. 548,280B1 are common in that the bottom faces of a plurality of triangular-pyramidal reflective elements serving as cores of entrance and reflection of light are present on the same face, a pair of elements faced each other respectively have analogous shapes, and heights of elements are equal to each other. Every retroreflective sheeting constituted of a triangular-pyramidal retroreflective element whose bottom face is present on the same face is inferior in entrance angularity, that is, every retroreflective sheeting has a disadvantage that retroreflective brightness rapidly decreases when the entrance angle of light to the triangular-pyramidal retroreflective elements increases.
As an attempt for improving the observation angularity, the official gazette of Japanese Patent Laid-Open No. 143502/1988 by Appeldorn et al. discloses an attempt that to make a triangular-pyramidal cube-corner prism die by cutting the surface of a flat plate with a diamond cutter or the like from three directions and forming V-shaped grooves intersecting at one point, a plurality of triangular-pyramidal retroreflective element groups are formed by slightly tilting symmetric faces of V-shaped grooves from the direction vertical to the flat plate and slightly deviating a cutting angle from a normal value, and cutting the V-shaped grooves, and a slight divergence is provided for reflected light of a cube-corner retroreflective sheeting formed by the die according to the element groups. The pair of reflective elements thus obtained have substantially analogous shapes and the elements substantially having the same height share a base and form a shape in which the elements rotate by 180.degree. from each other.
A retroreflective sheeting obtained through the method proposed by Appeldorn et al. can be a sheeting whose entrance angularity and observation angularity are improved to a certain extent. However, very complex operations are required together with a very-high accuracy and skill.
Moreover, a retroreflective element assembly is also publicly known which includes an asymmetric retroreflective element pair in which three-directional V-shaped grooves do not intersect at one point.
For example, the official gazette of International Patent Publication No. 94/14,091 (WO94/14091) by Gubela discloses a unique retroreflective body and its forming method in order to providing wide angularity for retroreflected light by decreasing the non-retroreflective surface of the retroreflective body. The retroreflective body is constituted by setting a hexagonal pyramid whose bottom face is an equilateral hexagon (A.sub.0 -D.sub.1 -E.sub.1 -B.sub.0 -E.sub.2 -D.sub.2) to the central portion of the bottom face of a rhombus formed when two-directional V-shaped grooves shown in FIGS. 5 and 6 intersect each other at an angle of 60.degree. and symmetrically arranging two equilateral triangular pyramids whose bases are equilateral triangles (D.sub.1 -C.sub.1 -H.sub.1 and D.sub.2 -C.sub.2 -E.sub.2 ) and whose heights are equal to each other. Among six lateral faces of the central hexagonal pyramid, three lateral faces (faces d.sub.1, d.sub.2, and d.sub.3 and faces e.sub.1, e.sub.2, and e.sub.3) every other one form two sets of retroreflective-prism lateral faces perpendicularly intersecting on each extended face.
Therefore, in the case of the retroreflective body described in the official gazette of Gubela, another-directional V-shaped grooves (D.sub.1 -E.sub.1 and D.sub.2 -E.sub.2) do not pass through the apex of a rhombus (A.sub.0 -C.sub.1 -B.sub.0 -C.sub.2) formed by four V-shaped grooves (A.sub.0 -C.sub.1, B.sub.0 -C.sub.2, and A.sub.0 -C.sub.2, B.sub.0 -C.sub.1) and an offset value from the apex (H.sub.0) of the another-directional V-shaped grooves is equal to 25% of the length of the longer diagonal line of the rhombus (that is, in FIGS. 5 and 6, intervals between C.sub.1 -C.sub.1, E.sub.1 -E.sub.1, B.sub.0 -H.sub.0, E.sub.2 -E.sub.2, and C.sub.2 -C.sub.2) shown by dotted extension lines are equal to each other and are 1/4 the interval between C.sub.1 -C.sub.1 and C.sub.2 -C.sub.2). Thereby, a pair of equilateral triangular pyramids which have the same height and which are symmetric and one hexagonal pyramid whose bottom face is an equilateral triangle (A.sub.0 -D.sub.1 -E.sub.1 -B.sub.0 -E.sub.2 -D.sub.2) are formed in the rhombus. Moreover, no description or suggestion about a retroreflective-element assembly specified by the present invention is present in the official gazette.
Moreover, for the light incoming from the bottom-face direction of the Gubela's hexagonal pyramid to retroreflect, it is necessary that the light reflected from a first entrance face repeats reflection only on the above faces every other one. If a second or third reflective face is a face other than the above faces, the light does not retroreflect but it passes through the face or diverges. Therefore, a certain effect is expected on improvement of observation angularity due to spread of reflected light. However, improvement of entrance angularity is not expected at all but entrance angularity is rather forced to deteriorate.
Moreover, the official gazette of International Patent Publication No. WO95/11,470 (Specification of U.S. Pat. No. 5,600,484), official gazette of International Patent Publication No. WO95/11,463 (Specification of U.S. Pat. No. 5,721,640), and official gazette of International Patent Publication No. WO95/11,465 (Specifications of U.S. Pat. Nos. 5,557,836 and 5,564,870) disclose a retroreflective body constituted of a retroreflective-element assembly enclosed by asymmetric V-shaped grooves whose one-side wall has an angle almost vertical to or close to a base bottom face and its manufacturing method inn order to improve retroreflectivity and wide angularity.
As disclosed in the above official gazettes of International Publications, the retroreflective body by Benson et al. is cut so that another-directional tilted V-shaped groove does not pass through the intersection between rhombic base shapes formed by two-directional tilted V-shaped grooves and can be constituted of various reflective elements including elements having no retroreflectivity by changing the intersection angle, depth, V-shaped-groove angle, number of grooves, and degree of V-shaped-groove tilt of the two-directional V-shaped grooves and the offset position, number of grooves, depth, V-groove angle, and degree of V-shaped-groove tilt of the another-directional V-shaped groove.
Moreover, it is shown that because the retroreflective body by Benson et al. is an asymmetric V-shaped groove in which a V-shaped groove lateral face is tilted almost vertically to the base bottom face, a midway shape whose base is rhombic formed by two-directional V-shaped grooves passes through the shape asymmetric to right and left shown in FIG. 2 and reflective lateral faces formed of the midway shape include the faces a.sub.2 and b.sub.2 in FIG. 2. Moreover, a midway shape according to the prior art is formed of symmetric V-shaped grooves as shown in FIG. 1, reflective lateral faces to be formed are a pair of faces (faces a.sub.1 and b.sub.1 and faces a.sub.2 and b.sub.2).
In the case of the assembly of these reflective elements, optical axes of reflective elements faced each other at the both sides of a V-shaped groove are oriented toward the same direction because of the shape of the assembly. For example, even in the case of reflective elements whose optical axes tilt, the optical axes tilt in the same direction. Therefore, a slight improvement of observation angularity is expected in accordance with spread of reflected light due to variety of types of reflective elements. From the viewpoint of entrance angularity, the reflective-element assembly has a very high directivity and thereby, superior entrance angularity is expected in an optical-axes, tilting direction. However, the assembly is unavoidably inferior in entrance angularity in other directions.
Problems to Be Solved by the Invention
In general, the following are requested for a triangular-pyramidal cube-corner retroreflective sheeting as basic optical characteristics: high-brightness characteristics such as height (magnitude) of reflection brightness represented by reflection brightness of light incoming from the front of the sheeting and wide angularity. Moreover. three performances such as observation angularity, entrance angularity, and rotation angularity are requested for wide angularity.
As described above, any retroreflective sheeting constituted of conventionally-publicly-known triangular-pyramidal cube-corner retroreflective elements has a low entrance angularity and does not have observation angularity to be satisfied. However, the present inventor et al. unexpectedly find that it is possible to improve the entrance angularity of a retroreflective sheeting constituted of a triangular-pyramidal reflective element in which V-shaped grooves having substantially-symmetric cross sections intersect each other and thereby, a pair of triangular-pyramidal cube-corner retroreflective elements partitioned by three lateral faces (faces a.sub.1, b.sub.1, and c.sub.1 ; faces a.sub.2, b.sub.2, and c.sub.2 ; . . . ) are arranged so as to protrude to one side on a common bottom face (S-S') in a closest-packed state, faced lateral faces (faces c.sub.1 and c.sub.2) of this pair of triangular-pyramidal retroreflective elements share a base (x) and are paired, the bottom face (S-S') is a common plane including the base (z and z) of one-side lateral faces (faces a.sub.1 and a.sub.2) and bases (y and y) of the other-side lateral faces (faces, b.sub.1 and b.sub.2), this pair of triangular-pyramidal retroreflective elements have faced lateral faces (faces c.sub.1 and c.sub.2) different from each other in shape, and heights from the base bottom face (S-S') up to apexes are different from each other.